Automated systems using speech recognition and text-to-speech synthesis techniques are familiar to persons calling over the telephone, for example, to a company call center. The design of the dialog in such application is relatively straightforward, since the designer is limited to a set of activities that (1) make sense in the context of the call (one doesn't order pizza when calling a bank); and (2) that are requests that the system can fulfill (e.g., providing an account balance). With the increasing use of mobile phones, however, it is possible to design a system that is more entertaining or informative, yet still allows the caller to impact the flow of the call by what they say. Such dialogs can potentially cover almost any subject, and significant creativity is required to keep the caller engaged and feeling that responses of the computer are relevant to what they said. Persons who write dialog designs for call centers are not chosen for their entertainment skills, and the tools available to them are not designed for the unstructured flow of an entertainment-focused system.
On the other hand, script writers for traditional entertainment channels such as television create dialog scripts that include a linear progression of dialog lines that the characters in the script take turns delivering. The classic creative writer gets to determine both sides of the conversation, and there is no “branching,” that is, the writer doesn't have to write a number of alternative replies that cover anything the independent speaker (the caller) may say. The linear technique is effective for applications in which the scripts are linear and predictable, such as for movies, plays and the like.